1. Field of the Invention
The invention relates to a sheathed piezoelectric actuator module, bathed for instance by liquid media, and to a method for producing it.
2. Description of the Prior Art
It is known per se that for constructing the aforementioned piezoelectric actuator module, piezoelectric elements can be inserted in such a way that by utilizing what is known as the piezoelectric effect, control of the needle stroke of a valve or the like can be performed. Piezoelectric layers of the piezoelectric elements are constructed from a material having a suitable crystalline structure, in such a way that upon application of an external electrical voltage, a mechanical reaction of the respective piezoelectric element takes place, which as a function of the crystalline structure and of the regions where the electrical voltage is applied represents a compression or tension in a predeterminable direction. Such piezoelectric actuators formed of piezoelectric elements are suitable for instance for applications in which reciprocating motions take place in the presence of strong actuating forces and at high pulse frequencies.
For example, one such piezoelectric actuator module is known from German Patent Disclosure DE 10026005 A1, as a component of a pink, and can be used for triggering the nozzle needle in injectors for injecting fuel into the combustion chamber of an internal combustion engine. In this piezoelectric actuator module, a stack of a plurality of piezoelectric elements, coupled to one another electrically and mechanically, is constructed in such a way that the stack is held by initial tension between two stops via an actuator foot and an actuator head. Each piezoceramic piezoelectric layer is fixed between two inner electrodes, by way of which an electrical voltage can be applied from outside. Because of this electrical voltage, the piezoelectric elements then each execute short reciprocating motions in the direction of the potential gradient, and these motions are added together to make the total stroke of the piezoelectric actuator. This total stroke is variable by way of the level of the voltage applied and can be transmitted to a mechanical final control element.
Such known arrangements are often used as so-called common rail systems for introducing fuel into direct injection diesel engines. In these known systems, the injection pressure can be adapted to the load and rpm of the engine in a simple way.
These common rail injectors can be constructed in such a way that a nozzle needle controlled indirectly by the piezoelectric actuator is present, and the piezoelectric actuator module is surrounded directly or indirectly by the pressure of the fuel, and between the nozzle needle and the piezoelectric actuator module, only a hydraulic coupling chamber is provided. It is important that the relatively vulnerable piezoelectric actuator be protected, in the interior of a retaining body, from an overly high pressure load and from fluctuations in pressure or temperature. In addition, protection against mechanical shock or pressure is necessary, particularly with a view to electrical insulation and protection against moisture (diesel, water, RME or other electrically conductive substances), particles, or floating materials.
To achieve electrical and mechanical insulation of the piezoelectric actuator, sheath of the piezoelectric actuator is often proposed. From German Patent Disclosure DE 10230032 A1, to avoid the disadvantages described, an arrangement with a piezoelectric actuator in bathing media is known in which the piezoelectric elements are potted in a variable-shape insulation compound, which in turn is placed in a housing jacket that is solidly closed off from the medium laterally and at the upper and lower ends.
Viewed per se, it is known from a number of applications that with an elastic plastic extrusion coating, the piezoelectric actuator, for instance in a direct-switching pink in which the piezoelectric actuator module is exposed directly to the high pressure of the fuel inlet, can be protected against the harmful factors mentioned above. In these direct-switching pinks, only a mechanical coupler with a hydraulic booster is located between the piezoelectric actuator module and the nozzle needle, so that the needle motion follows the motion of the piezoelectric actuator in a directly boosted fashion, and in this case two-stage boosters are also possible.
In these known arrangements, the problem still exists of sealing off the coating material, as a rule plastic, from the steel attachment parts, such as the actuator head and foot on the face ends of the piezoelectric elements of the piezoelectric actuator. Either this sealing can usually be produced only in adhesive fashion, or the attachment is subject to the strength of adhesion of the coating materials to the steel parts.
The arrangements known for indirect control of the nozzle needle that have a piezoelectric actuator module in the low-pressure region of the fuel have metal sleeve sealing, which can be used here as a reciprocation- and temperature-compensating element. However, these arrangements cannot be adopted to applications with direct control of the nozzle needle. Because of the construction with a piezoelectric actuator module in the high-pressure region in the case of direct control, a rigid metal sleeve seal would not withstand the high system pressures and the longer reciprocating motions of the piezoelectric actuator, given the available space.